MITOSIS
Mitosis - The division of cells to produce genetically identical daughter cells.
-
Happens in multi-cellular organisms
-
Division and replication of nuclear material
-
Has 2 phases:
-
INTERPHASE - Normal Cell Activity
-
MITOTIC PHASE - Division of cell
-
IMPORTANCE OF MITOSIS
​
a) Reproduction (e.g. amoeba)
b) Growth & Development (e.g. embryos)
c) Tissue Renewal (e.g. RBCs)
Genome - Complete genetic information of an ORGANISM (packaged in chromosomes).
Chromatin - Unorganized form of DNA (loosely assorted DNA threads).
Nucleotide - Phosphate + sugar + base
Nucleosome - Every 200 nucleotide pairs, the DNA wraps itself twice around a group of 8 histone proteins.
Coiling - DNA coils around protein
Super-coiling - Nucleosomes start coiling around each other
*** Chromatin condenses and coils around protein to form chromosomes during replication.
PROKARYOTES
- Single strand of DNA
- Single stranded
EUKARYOTES
- 1000x more DNA strands than prokaryotes
- Double stranded
​
​
​
​
HUMANS
- 46 chromosomes
- 23 nearly-identical homologous pairs
(1 from each parent)
​
​
CHROMOSOMAL PAIRS
22 - Autosomal Pairs
1 - Sex Pair
KARYOTYPE
Figure 1. Chromosomes before and after replication
Figure 2. Chromosomes & chromatids
Homologous chromosomes
- Look alike
- Control the same traits
- Inherited from each parent
- May code for different forms of each trait.
Non - Homologous chromosomes
- Look different
- Control different traits
SEX CHROMOSOMES
- Distinct from each other in their characteristics
- Represented as X & Y
-
XY - Male
-
XX - Female
- In a diploid cell (normal functioning cell - 2n), the chromosomes occur in pairs. The 2 members of each pair are called homologous chromosomes or homologues.
2 disks of proteins called kinetochores are bound to the centromere. Microtubules attach to these kinetochores and pull them away during mitosis.
​
Faults in kinetochores lead to mutations and faulty cell division (chances increase with age).
Figure 3. Chromosome & Kinetochores
Process of Mitosis
INTERPHASE
-
G1 - Increase in cell size and mass (to adult cell)
-
S - Duplication of DNA
-
G2 - Replication of all organelles
THE MITOTIC SPINDLE
​
-
Aster - Connected to cell membrane, radial array of microtubules from centrosomes.
-
Centrosomes [2 pairs of centrioles] (microtubule organizing center)
-
Spindle microtubules
-
Kinetochore microtubules - attach to chromosome
-
Non-kinetochore microtubules - attach to opposite incoming microtubules.
-
CYTOKINESIS
-
Cleavage forms and two daughter cells are formed
-
IN PLANTS - Cell wall forms between the two daughter cells at the 'cell plate'.
PROPHASE
-
Nucleus increases in size
-
Early mitotic spindle fibre forms
-
Nuclear envelope disintegrates (prometaphase)
METAPHASE
-
All chromosomes align in center of cell at metaphase plate
-
All spindle fibres attach to chromosomes/other spindle fibres
-
Each chromosome must be attached to microtubules of both centrosomes.
ANAPHASE
-
Chromosomes are pulled apart
-
Kinetochore microtubules shorten and constrict, pulling chromosomes to two ends of cell.
-
Non-kinetochore microtubules elongate and cause cell to elongate.
-
Cell opens up from center and starts squeezing from center.
TELOPHASE
-
Nuceli of cell starts to develop from fragments of broken nuclear envelope of parent cell
-
Mitotic spindle fibre breaks down
-
Nucelolous reforms first
-
Chromosomes become chromatim
Figure 4. Mitosis
IMPORTANCE OF CYTOSKELETON IN MITOSIS
​
1. Microtubules - Controlling movement of chromosomes and allowing cell to elongate.
​
2. Intermediate filament - Allows cell to elongate and hold organelles in place.
​
3. Actin filament - forms constriction belt and allows for cleavage furrow.
APOPTOSIS - How cell self-destructs (lysosomes)
​
*If DNA is damaged, cells lose ability to control rate of cell division (mutation)
*Cell division at an uncontrolled rate is cancer
​
TUMORS
Benign - Not cancerous, do not spread to other parts of body.
Malignant - Canerous, cells break lose and can invade and destroy healthy tissue in other parts of body (metastasis - when cancer infects healthy tissue)